1. Field of the Invention
The present invention relates to a short arc type ultra-high pressure discharge lamp used as a light source for a projection apparatus that uses a microdevice mirror or a liquid crystal display apparatus.
2. Description of the Related Art
Good color rendition and high brightness are required of light sources for projection apparatuses that use microdevice mirrors and liquid crystal display apparatuses.
To improve color rendition, metal halide lamps filled with various light-emitting metals were used in the past, but in recent years there have been demands for lamps that have both yet better color rendition and yet higher brightness, and hence short arc type ultra-high pressure discharge lamps that utilize the vapor pressure of mercury and have a very high pressure inside the arc tube have come to be used.
FIG. 5 shows such a short arc type ultra-high pressure discharge lamp.
The short arc type ultra-high pressure discharge lamp comprises a quartz glass arc tube 1, and quartz glass seal portions 2 that are formed on both sides of the quartz glass arc tube 1; a pair of electrodes 3 are disposed opposite one another inside the arc tube 1.
A metal foil 4 is connected to one end of each electrode 3, with the metal foil 4 and part of the electrode 3 being hermetically embedded in the respective seal portion 2.
Moreover, an external lead rod 5 is connected to each metal foil 4, with the external lead rod 5 extending out to the outside from the respective seal portion 2.
The inside of the arc tube 1 is filled with at least 0.15 mg/mm3 of mercury.
By filling with at least 0.15 mg/mm3 of mercury in this way, when the lamp is turned on, the mercury vaporizes inside the arc tube to an extremely high pressure of 1.5xc3x97107 Pa or more, whereby spreading of the arc is suppressed, and hence good color rendition and high brightness are realized.
However, with such a short arc type ultra-high pressure discharge lamp, there has been a problem that the pressure inside the arc tube 1 becomes extremely high when the lamp is turned on, and the phenomenon of xe2x80x98foil floatingxe2x80x99 thus occurs in which the metal foil 4 embedded in each seal portion 2 breaks away from the quartz glass constituting the seal portion 2, and hence the seal portion 2 is damaged.
The reason for this is that the molybdenum foil constituting the metal foil 4 and the quartz glass have a different expansion coefficient to one another, and hence tiny gaps are formed between the metal foil 4 and the quartz glass constituting the seal portion 2 during manufacture; the gas at extremely high pressure inside the arc tube 1 flows into these gaps, and thus stress is generated that forces the metal foil 4 and the quartz glass apart.
Furthermore, because the molybdenum foil constituting the metal foil 4 and the quartz glass have a different expansion coefficient to one another, when the lamp is turned on the metal foil becomes hot and tries to expand, but the quartz glass does not expand so much; this difference in forces is manifested as thermal stress, and hence cracks may arise, or microcracks in the seal portion 2 that arise during the manufacturing process may be caused to grow, thus damaging the seal portion 2.
The present invention has been produced to resolve problems such as the above, and it is an object thereof to provide a short arc type ultra-high pressure discharge lamp according to which seal portions are not damaged even if the pressure inside the arc tube becomes high.
A short arc type ultra-high pressure discharge lamp defined in claim 1 is a short arc type ultra-high pressure discharge lamp in which a pair of electrodes are disposed inside an arc tube that comprises quartz glass, seal portions are formed that comprise quartz glass and extend to both sides of the arc tube, and at least 0.15 mg/mm3 of mercury is filled into the arc tube; wherein a metal foil, and part of each electrode connected to this metal foil, are embedded in each of the seal portions, and metal granular lumps are protrusively provided on surfaces of the metal foil embedded in each of the seal portions.
A short arc type ultra-high pressure discharge lamp defined in claim 2 is the short arc type ultra-high pressure discharge lamp according to claim 1, wherein the metal granular lumps comprise any one of tungsten, tungsten compounds, molybdenum, molybdenum compounds, and compounds of tungsten and molybdenum.
A short arc type ultra-high pressure discharge lamp defined in claim 3 is the short arc type ultra-high pressure discharge lamp according to claim 2, wherein the granular lumps have a thickness in a range of 0.001 to 1 xcexcm, and the granular lumps cover the metal foils at a coverage of not more than 80%.